The proposed study aims at the analysis of control of gene expression during the transition from proliferating precursor cell to postmitotic differentiation in established myogenic cell lines which can proliferate apparently indefinitely without losing their capacity to differentiate. Attempts will also be made to elucidate the etiology and nature of the changes in the cells involved in the formation of these lines. The differentiation of cells or primary muscle cultures and myogenic lines will be controled by changing the composition of the nutritional media. The temporal changes in the synthesis and processing of mRNA will be correlated with the morphological changes and the appearance of the corresponding proteins. The influence of different environmental and genetic factors on the successful establishment of myogenic cell lines will be investigated.